1. Field Of The Invention
The invention relates to a method for controlling the operating temperature of a soldering iron to a predetermined value with the aid of a temperature sensor arrangement contained in the soldering iron which, depending on the temperature measured, turns a heating element of the soldering iron on or off.
2. Description Of The Prior Art
In known methods of this kind the temperature sensor arrangement consists of a single temperature sensor disposed in the region of the heating element. By this method the temperature of the soldering iron in the region of the heating element can be measured and also regulated properly. But for satisfactory operation of the soldering iron it is more important to aim at the temperature of the soldering tip. It is, however, not possible for practical reasons to install a temperature sensor directly in the soldering tip, because the temperature sensor would considerably impede the required good heat transfer to the area being soldered.
It was therefore not possible until now to keep the temperature of the soldering tip constant independently of the heat flow emanating from the soldering tip. In particular it would happen that if there was much heat transfer from the soldering tip, the soldering tip temperature decreased so sharply that satisfactory soldering was no longer ensured.
It is, therefore, the object of the present invention to develop a method and an apparatus of the initially mentioned kind in such a way that, independently of the magnitude of the heat transfer from the soldering iron tip, the temperature of the soldering iron tip is always maintained constant.
For the solution of the problem there is proposed a method according to the invention which is characterized in that for stabilization of the temperature at the soldering tip independently of the hat flow from the soldering tip, the soldering tip body is conceived in its longitudinal extent as a controlled system on which several temperature sensors are arranged spaced from each other, and that the parameters picked up by the temperature sensors, such as temperature, rate of temperature change, temperature rise, are supplied to a computer which, via a mathematical model, determines the soldering tip temperature not measured and keeps it constant independently of the heat flow.
Thus it is an essential feature of the present invention that the soldering tip body is conceived as a controlled system in its longitudinal extent, on whose length several temperature sensors are arranged spaced from each other. Each temperature sensor (pick up) furnishes a measured quantity proportional to the temperature. The temperature sensors are located at points of different temperature level.
From the measured values the non-measurable temperature at the tip of the soldering iron is inferred by means of a mathematical model. To this end, quantities such as temperature, rate of temperature change, temperature rise and the like are evaluated.
For regulation there are used measured or calculated parameters, namely insofar as the system is controlled either to the tip temperature value calculated by the model or to other values determined from the measured quantities.
Thereby the essential advantage is achieved that now the actual soldering tip temperature can always be calculated in the computer independently of the particular load (heat transfer at the soldering tip) and accordingly the output of the heating element can be readjusted.
To establish an appropriate control model, actually any number of temperature sensors may be arranged spaced from each other along the soldering tip body. However, for measuring and circuitry reasons it is preferred to use as few temperature sensors as possible. For this there are several options, among which the use of two mutually independent temperature sensors is preferred.
To make the model of the controlled system used by the computer as accurate as possible, it is preferred to arrange the first sensor as close to the soldering tip as possible, but without impeding the heat transfer from the soldering tip.
In this connection it is preferred to design this sensor as a thermocouple, being that it has heretofore been possible only with thermocouples to arrange them near the tip so that on the one hand they do not adversely alter the tip geometry and on the other hand do not impede the heat transfer.
Suitable as material for a thermoelement has been found to be a Ni-Cr-Ni material. Now only one additional temperature sensor is needed, which is disposed spaced from the first temperature sensor, designed as thermocouple, in the direction of the heating element.
It is then preferred to use the heating element itself as temperature sensor, because thereby in turn the cost of circuitry is substantially reduced.
In fact, if according to the invention one uses as heating element a ceramic tube which is provided with an electroconductive, current-carrying coating, the type of coating can be chosen so that the coating acts as a PCT resistor. Thus, by measuring the voltage drop at the heating element and by measuring the current flow through the heating element, the effective resistance of the heating element can be determined, and therefrom the effective heating element temperature.
Use of a PCT material as heating element has the further advantage that with a cold heating element there is a very low electric resistance, so that a very rapid heating takes place, so that thereby also quick heat flow changes at the soldering tip can be compensated quickly.
The said measured values of these two temperatures sensors are supplied to the analog input of a digital computer, which internally processes these values via an analog/digital computer, which internally processes these values via an analog/digital converter and feeds them as measured quantities into its internally established computation program.
The computation program determines therefrom the soldering tip temperature not measured. For this purpose quantities such as temperature, rate of temperature change, temperature rise and so forth are evaluated.
Certain process data such as mode of operation, desired values, etc. can be entered via a keyboard or made visible via a display. Upon turnoff of the equipment, various parameters, e.g. desired values, can be stored in a non-volatile memory.
For determining the temperature in the soldering iron the following parameters are measured:
1. the voltage which the thermocouple delivers in the soldering iron, PA1 2. a voltage proportional to the terminal point temperature, by means of a measuring resistor, PA1 3. the voltage present at the heating element, PA1 4. a voltage proportional to the heating element current, by means of a measuring resistor. (Rm)
These quantities are amplified and resolved in the A/D converter of the microprocessor with 8 or 9 bits. From the digital values thus obtained Hk and TH.e can be calculated and used for control in the above stated manner.
The soldering iron is operated on contactproof AC voltage. As power control element a semiconductor switch (Triac) is used. The power control element is switched on or off always in the zero crossing of the AC voltage. For synchronization the microprocessor is supplied a square signal whose edge falls with the zero crossing of the load voltage.
Communication with the microprocessor is possible via a serial interface.
The subject of the present invention results not only from the subject of the individual claims but also from the combination of the individual claims with one another. All data and features disclosed in the documents--including the summary--in particular the physical realization illustrated in the drawings are claimed as essential to the invention insofar as they individually or in combination are novel relative to the prior art.